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What effect will dry summers like the one in 2018 have on nature in Switzerland? Experts at the WSL focus on the water balance and on the forest as a resource and place for recreation. It looks as if there could well be major changes, but not at all levels. Manfred Stähli (S) and Andreas Rigling (R) in conversation with Roland Fischer.

Mr Stähli, Mr Rigling, summer 2018. How did you find it from a technical point of view?S — For a start, we should mention the winter, which was very snowy. That actually created the perfect conditions as there was a lot of water in the ground.R — But then April came along, which was decisive for the vegetation. The weather was constantly dry from then on. In terms of summer heat, 2018 is actually comparable to 2003, which was previously used as a yardstick for everything.S — That's true; it matched the summer heat of 2003, which had been unparallelled up until then. But this summer had some elements that were even more extreme than then. For example, the runoff in watercourses in 2018 was lower in many places than it had been in 2003. There had also been a sharp decline in groundwater, for example in the Thur area.

WSL experts: Andreas Rigling (left) and Manfred Stähli

And fifteen years later, we have had another extreme. Is it fair to say that the extremes are mounting up?S — Yes, things are heading that way. We have now had four very dry years within a short time.

What impact has this had on woodland?R — I have been studying the damage caused by drought on woodland for over 20 years. We keep observing what are known as mortality peaks. But these don't necessarily come directly after extreme years. We have found that they occur more often when several dry years follow each other. The forest can cope with one dry year, but when the next one is dry again, it can become critical.

Andreas Rigling, WSL member of the directorate and professor in the Department of Environmental System Sciences at ETH Zurich, heads the Forest Dynamics research unit. Manfred Stähli, Head of the Mountain Hydrology and Mass Movements research unit, also studies the impact of climate change on water resources.

Why is that?R — The trees take emergency steps in those sorts of dry seasons. They close their stomata to protect themselves from excessive transpiration and reduce their entire metabolism. It's a good strategy, but not for too long: a tree produces much less sugar in years like that. This can be seen in the narrow annual rings, for example. It lives off the reserves, but eventually they become depleted.S — It is similar for the water balance as a whole to some extent. There are also so-called memory effects. The ground-water table has not recovered completely yet, for example, and this will take some time. However, this is not a problem in the medium term, as the groundwater is replenished again and again over several years.

How do you envisage that? Is there a big lake below Switzerland?S — No, the underground topology is complex. The groundwater is not evenly distributed everywhere, it depends largely on the geology. This means that localised shortages can occur.R — Groundwater is not crucial for wood-land. It is more a matter of the ratio of precipitation, sunlight and water storage in the topsoil.

What do we know about it, are there any trends?S — Yes, water availability is changing. There are likely to be wetter winters and drier summers. We also expect to see more heavy rainfall.R — The history of the founding of WSL has a lot to do with extreme events such as floods. And that's what is increasingly engaging us on many different levels.S — It has become a common issue.

How dramatic is this going to be for woodland?R — That depends on your perspective. While woods will not disappear, we will see a shift in species to a greater or lesser extent. Oaks, for example, cope much better with drought, but spruce and beech may find it difficult to hold their own on the Central Plateau. While oak forests can also be used for recreation, the timber sector is also facing a major shift.

And what are the prospects on the water balance side?S — The NRP 61 on sustainable water use, which was recently completed, came to the following conclusion: there will be more shortages locally, but overall we will have enough precipitation, even for the extreme climate scenarios

So, this won't be a problem for agriculture?S — It will certainly be a challenge. Farmers will have to water more, that is foreseeable, and possibly adapt their crops.R — Yes, there is also a crucial difference in this adaptation. The agricultural sector adapts quickly and can change from one year to the next. However, woodland operates over much longer periods of time, from decades to centuries. And the timber industry is geared towards the spruce and will be forced to rethink. The early leaf fall from beech trees and massive infestation by bark beetles in some areas in 2018 was probably an eye-opener, even for the foresters.

What role does the WSL play, and what response is needed to such extreme years with specific research projects?S — Together with MeteoSwiss, we have developed a hydrological 30-day forecast for Switzerland. Preventative measures can be taken with a monthly outlook, even if they are only forecasts.R — These years always present us with a unique opportunity. We learn a lot about the processes on the basis of climatic extreme years. We have stepped up our monitoring and selected 1000 trees throughout Switzerland. We can now see how affected trees will “digest” the extreme event of 2018. We investigate the fundamental relationships between the reactivity and adaptability of our trees in order to answer pressing practical questions. Do trees that lose their leaves early have to be felled or can they recover? There is simply a lack of empirical information because those sorts of summers have been so rare so far. We can provide support in this respect.